1. Field of the Invention
The instant invention relates to high frequency circuits operating in microwave band, millimeter wave band, and particularly to a configuration of a microwave integrated circuit (MIC) or a monolithic microwave integrated circuit (MMIC). The invention particularly relates to a microwave filter, which can be employed in the MIC or the MMIC.
2. Description of the Related Art
In these years, it becomes urgent to increase communication channel numbers by rapid growth of informations demanded in the field of information communication. Therefore, the practical communication systems operating in the microwave/millimeter band, which were not used an earlier time, are now being promoted rapidly. As for the RF portion of the microwave communication apparatus, RF circuits such as a RF generator, a RF synthesizer, a RF modulator, a RF power amplifier, a RF low-noise amplifier, a RF demodulator, and a RF antenna are incorporated therein, generally. For the communication apparatus, the achievement of superior electric characteristics and miniaturized size is the principal objective of the research and development. For the achievement of the miniaturization of a RF portion, it is necessary to integrate RF circuitry. Therefore, the implementations of the MICs or the MMICs are considered to be effective.
Integration of the RF circuitry on a semiconductor chip has been developed with the rapid evolution of the semiconductor integration techniques. The circuitry merged in a semiconductor chip has been changed from an earlier discrete active element to a functional circuit block, which can serve as one of the RF circuitry of the communication apparatus. Further, the degree of on-chip integration has increased so that plural functional circuit blocks are merged into one semiconductor chip. In the MIC or the MMIC, active elements such as high electron mobility transistors (HEMTs), hetero junction bipolar transistors (HBTs), Schottky gate field effect transistors (MESFETs) as well as the passive elements such as capacitors (Cs), inductors (Ls), and resistors (Rs) are integrated. To implement the high frequency circuits, being merged into the MMIC, filters are often employed for the purpose of removing unnecessary signals from a targeted signal. In the RF circuitry, microwave filters are often employed for removing unnecessary signals from the RF signals, which are scheduled to be transferred into the IF circuitry.
However, earlier microwave filters have manifested poor performance, showing high transmission loss in a frequency range higher than cut-off frequency fc. The poor performance is ascribable to the phenomena that high frequency current is easy to flow an edge of filter, and thereby the current crowding is generated to dissipate high frequency powers.